Liquid chromatograph mass spectrometers (LC/MS) are formed of a liquid chromatographic unit (LC unit) for separating a liquid sample into components, each of which is eluted, an ionization chamber (interface unit) for ionizing a sample component that has been eluted from the LC unit and a mass spectrometric unit (MS unit) for detecting ions that have been introduced from the ionization chamber. In such an ionization chamber various ionization techniques for ionizing a liquid sample are used and, in particular, atmospheric pressure ionization methods, such as the atmospheric pressure chemical ionization method (APCI) and the electrospray ionization method (ESI), are widely used.
Concretely, in the APCI the tip of a nozzle connected to the end of the column in the LC unit is placed so as to be directed toward the inside of the ionization chamber and, at the same time, a needle electrode is provided in front of the tip of the nozzle. Thus, a droplet of a sample that has been atomized through heating in the nozzle is made to chemically react with carrier gas ions (buffer ions) generated through corona discharge from the needle electrode so as to be ionized. In addition, in the ESI the tip of the nozzle connected to the end of the column in the LC unit is placed so as to be directed towards the inside of the ionization chamber and, at the same time, a high voltage of approximately 5 kV is applied to the tip portion of the nozzle so that a strong non-uniform electrical field is generated. As a result, the liquid sample is subjected to charge separation by means of the electrical field and, thus, is torn apart through Coulomb attraction so as to be atomized. As a consequence, the solvent in the droplet of the sample evaporates through contact with the surrounding air and, thus, gas ions are generated.
As described above, in the APCI and ESI, the liquid sample is ionized in a state that is close to atmospheric pressure and, therefore, a structure is adopted such that a middle chamber is provided between the ionization chamber and the MS unit so that the degree of vacuum can be increased step by step in order to maintain a difference in pressure between the ionization chamber in a high pressure state (that is, a state close to atmospheric pressure) and the MS unit in a very low pressure state (that is, a state of a vacuum of a high degree) (see Patent Document 1).
The applicant has announced on Internet websites technologies for increasing the sensitivity in and the ease of maintenance of a liquid chromatograph mass spectrometer (see Non-patent Documents 1 to 4).
FIG. 8 is a schematic diagram showing an example of the structure of a liquid chromatograph mass spectrometer using an ESI method. FIG. 9 is a perspective diagram showing the liquid chromatograph mass spectrometer of FIG. 8.
The liquid chromatograph mass spectrometer has an ionization chamber 200, a mass spectrometric unit 50, a control chamber 160 and a housing unit 170.
The ionization chamber 200 has a compartment 210 in a triangular prismatic form made of aluminum, and the compartment 210 has an upper surface 210a, a first side, a second side, a third side and a lower surface. In addition, a circular opening is created in the center portion of the first side and the peripheral portion of the opening in the first side is attached in an airtight manner to the front surface of a first middle chamber 12 via an O ring made of rubber (not shown). Furthermore, a spray unit (ionization member) 15 is attached to the upper surface 210a of the compartment 210.
The control chamber 160 is located beneath the ionization chamber 200 and has a control chamber housing 161 in a rectangular parallelepiped form wherein a high voltage power supply 62 is provided inside of the control chamber housing 161. In addition, one end portion of a cable 63 is connected to the high voltage power supply 62 inside of the control chamber housing 161 and, at the same time, the other end portion of the cable 63 is connected to a connector 65, which is placed outside of the control chamber housing 161.
The liquid sample that has been separated into components in the LC unit is supplied to the spray unit 15 through a pipe 155. Though not shown, a nebulizing gas (nitrogen gas) is supplied to the spray unit 15 from a nebulizing gas supplying source through a pipe having a diameter of 3.2 mm for example. As a result, the liquid sample and the nebulizing gas are led to the spray unit 15 so as to be sprayed. At this time a connector 67 of a cable 64 connected to the spray unit 15 and the connector 65 of the cable 63 connected to the high voltage power supply 62 are connected to each other so that a high voltage of 5 kV is applied to the tip of the nozzle of the spray unit 15 from the high voltage power supply 62 and, thus, ionization can be achieved.
Though FIG. 8 shows a spray unit 15 for ESI, generally the spray unit 15 is removable from the compartment 210 and in the case wherein the APCI method is used, the spray unit 15 for ESI is removed and instead a spray unit for APCI where a needle electrode for discharge can be treated as a unit is attached to the compartment 210.
The mass spectrometric unit 50 is provided with a first middle chamber (vacuum introduction unit) 12 that is adjacent to the ionization chamber 200, a second middle chamber 13 that is adjacent to the first middle chamber 12 and a mass spectrometric chamber (MS unit) that is adjacent to the second middle chamber 13 in a manner wherein these chambers are connected to each with respective partitions there between.
The mass spectrometric unit 50 is provided with a housing 190 made of aluminum in a parallelepiped form of 15 cm×15 cm×90 cm, wherein a first ion lens 21 is provided inside the first middle chamber 12 and, at the same time, a discharge opening 31 for vacuum discharge by means of an oil-sealed rotary pump (RP) is provided at the bottom of the first middle chamber 12.
A heater block 20 having a built-in temperature adjustment mechanism (not shown) is fixed to the front surface of the housing 190, where a solvent removal tube 19 in a circular tubular form (an outer diameter of 1.6 mm and an inner diameter of 0.5 mm) is formed in the heater block 20. As a result, the inside of the compartment 210 and the inside of the housing 190 are connected to each other via the solvent removal tube 19. Therefore, the solvent removal tube 19 has such functions that removal of solvent and ionization are accelerated through heating and through collision when ions and fine droplets of the sample sprayed from the spray unit 15 pass through the inside thereof.
An octupole 23 and a focus lens 24 are provided inside the second middle chamber 13, and a discharge opening 32 for vacuum discharge by means of a turbo molecular pump (TMP) is provided beneath the second middle chamber 13. An entry lens 25 having a small hole is provided in the partition between the second middle chamber 13 and the mass spectrometric chamber 14 so that the inside of the second middle chamber 13 and the inside of the mass spectrometric chamber 14 are connected via this small hole.
A first quadrupole 16, a second quadrupole 17 and a detector 18 are provided inside the mass spectrometric chamber 14 and a discharge opening 33 for vacuum discharge by means of the turbo molecular pump (TMP) is provided at the bottom of the mass spectrometric chamber 14.
In this liquid chromatograph mass spectrometer, ions generated in the ionization chamber 200 are sent to the mass spectrometric chamber 14 after passing through the solvent removal tube 19, the first ion lens 21 within the housing 190 in the first middle chamber 12, a skimmer 22, the octupole 23 and the focus lens 24 within the second middle chamber 13, and the entry lens 25 in this order, where unnecessary ions are discharged by means of quadrupoles 16 and 17 so that only specific ions that have reached the detector 18 are detected.
Incidentally, in the above described liquid chromatograph mass spectrometer, the first ion lens 21 and the like require maintenance and, therefore, the structure allows the ionization chamber 200 to be treated as a unit so that the ionization chamber 200 can be moved between the maintenance position and the analysis position. For example, the ionization chamber 200 is rotatable by approximately 90° around the axis along the side between the first side and the front of the first middle chamber 12 in the vertical direction by means of a hinge (not shown). As a result, the user disconnects the connector 67 of the cable 64 connected to the spray unit 15 from the connector 65 of the cable 63 connected to the high voltage power supply 62 and removes the spray unit 15 in order to maintain the first ion lens 21 and the like and, after that, puts the ionization chamber 200 in the maintenance position and attaches the spray unit 15 for spectrometry followed by reconnection between the connector 67 of the cable 64 connected to the spray unit 15 and the connector 65 of the cable 63 connected to the high voltage power supply 62 and, then, puts the ionization chamber 200 in the analysis position.